What accounts for the similarities among all human beings, such as the universal capacity for language or loyalty to a family or clan? Evolutionary psychologists believe the answer lies partly in genetic dispositions that developed during the evolutionary history of our species.

Researchers in evolutionary psychology emphasize the evolutionary mechanisms that might help explain our human commonalities in a variety of areas, such as personality, emotion, sexual behavior, or reasoning. To read the messages from the past that are locked in our genes, we must first understand the nature of evolution itself. Evolution is basically a change in gene frequencies within a population, a change that typically takes place over many generations. As particular genes become more common or less common in the population, so do the characteristics they influence. These developments account for changes within a species. And when two populations within a species become geographically separated and must adapt to different conditions, eventually those two populations may evolve into two different species.

Why exactly do gene frequencies in a population change? During the division of the cells that produce sperm and eggs, if an error occurs in the copying of the original DNA sequence, genes can mutate. In addition, during the formation of a sperm or an egg, small segments of genetic material cross over from one member of a chromosome pair to another, exchanging places prior to the final cell division. As genes spontaneously mutate and recombine during the production of sperm and eggs, new genetic variations, and therefore potential new traits, keep arising.

But that is only part of the story. According to the principle of natural selection, first formulated in general terms by British naturalist Charles Darwin in On the Origin of Species (1859/1964), the fate of these genetic variations depends on the environment. Darwin did not actually know about genes, as their discovery had not yet been widely publicized, but he realized that a species' characteristics must somehow be transmitted biologically from one generation to the next.

The fundamental idea behind natural selection is this: In a given species living in a particular environment, some individuals with a genetically influenced trait tend to be more successful than others in finding food, surviving the elements, and fending off enemies—and are therefore better at staying alive long enough to produce offspring. As a result, their genes will become more and more common in the population, having been “selected” by reproductive success. Over many generations, these genes may even spread throughout the species. In contrast, those individuals whose traits are not as adaptive in the struggle for survival will not be as “reproductively fit”: They will be more likely to die before reproducing, and their genes—and the traits influenced by those genes—will therefore become less and less common, and may possibly even disappear.

Scientists debate how gradually or abruptly evolutionary changes occur and whether competition for survival is always the primary mechanism of change, but they agree on the basic importance of evolution. During the past century and a half, Darwin's ideas have been resoundingly supported by findings in anthropology, botany, and molecular genetics (Ruse, 2010). Scientists have actually watched some organisms evolving, such as microbes, insects, and plants. Some rapid evolutionary changes, in mammals as well as microbes, are due to human activity. For example, the horns of bighorn rams have been getting smaller because of trophy hunting, which removes animals with larger horns from the breeding population (Coltman et al., 2003). Researchers have even identified specific genes that account for evolutionary changes that have occurred in animals in the wild, such as the transformation of mice and lizards from light colored to dark colored (or vice versa) as the animals have migrated into different environments (Des Roches et al., 2013; Hoekstra et al., 2006). Evolutionary principles such as natural selection guide all of the biological sciences.

Although many evolutionary theorists have assumed that human evolution pretty much stopped thousands of years ago, scientists have found evidence of natural selection occurring occasionally in humans over just a few generations, in response to changing conditions. For instance, when Africans were brought to America as slaves, they had a genetic variation that protected them against malaria. Over time, as malaria became less of an environmental threat in the United States, natural selection slowly stopped favoring those with this genetic variation, and today it is less common among African Americans than it is among indigenous Africans (Jin et al., 2012). You can learn more about evolutionary psychology by watching the video Evolutionary Psychology 1.